Method and device for low-temperature air separation

ABSTRACT

A method and device for low-temperature separation of air in a distillation column system having a high-pressure column and a low-pressure column. The system includes a precolumn to which the first part of feed air is introduced and from which gaseous nitrogen product is removed and then heated in a heat exchanger. The precolumn includes a head condenser to which a liquefied portion of a second part of the cooled feed air is introduced. A gaseous fraction from the upper region of the precolumn is introduced to the system condenser and fluid formed in the system condenser is at least partially fed to the precolumn as return flow.

The invention relates to a method according to the preamble of Claim 1.

Methods and devices for the low-temperature separation of air are known,for example, from Hausen/Linde, Low-temperature technology, 2^(nd)edition 1985, Chapter 4 (pages 281 to 337).

The distillation column system of the invention includes a two-columnsystem (for example a classic Linde double column system) fornitrogen-oxygen separation having a high-pressure column and alow-pressure column, which are operatively interconnected for heatexchange. The operative interconnection for heat exchange between thehigh-pressure column and the low-pressure column, as a rule, is realizedby a main condenser in which head gas of the high-pressure column isliquefied against evaporating sump liquid of the low-pressure column. Inaddition to the columns for nitrogen-oxygen separation, the distillationcolumn system can comprise further devices, for example for producingother air components, in particular inert gases, for example producingargon which includes at least one raw argon column, or producingkrypton-xenon. Along with the distillation columns, the distillationcolumn system also includes the heat exchangers which are assigneddirectly to them and, as a rule, are realized as condenser-evaporators.

A “main heat exchanger”, in this case, serves for cooling the feed airin indirect heat exchange with return streams from the distillationcolumn system. It can be formed by one single or several heat exchangerportions which are connected in parallel and/or in series, for exampleby one or several plate heat exchanger blocks.

In a secondary condenser, which is also realized as acondenser-evaporator, oxygen removed in the liquid state from thelow-pressure column is evaporated at an only slightly increased oxygenpressure of between 1.5 and 6 bar, preferably between 2.7 and 4 bar.Part of the cooled feed air is liquefied against the evaporating oxygen.

A heat exchanger, in which a first condensing liquid stream enters intoindirect heat exchange with a second evaporating liquid stream, isdesignated as a “condenses evaporator”. Each condenser-evaporatorcomprises a liquefaction chamber and an evaporation chamber whichconsist of liquefaction passages or evaporation passages. The condensing(liquefying) of a first liquid stream is carried out in the liquefactionchamber and the evaporation of a second liquid stream is carried out inthe evaporation chamber. The evaporation chamber and the liquefactionchamber are formed by groups of passages which are operativelyinterconnected for heat exchange.

A method of the type mentioned in the introduction and a correspondingdevice are known from EP 1319913 A1 (=US 2003110796 A1). Pressurizednitrogen is also produced in this case from the distillation columnsystem; however, this is only possible to a limited extent because therethe respective flow is missing as reflux in the columns.

Within the framework of the invention, a method is sought that iscapable of generating large volumes of nitrogen along with the oxygen atthe slightly increased oxygen pressure and at the same time of keepingthe number of externally driven machines, in particular of thecompressors, which are not driven by a turbine of the method as small aspossible. The oxygen, in this case, is to be generated either as pureoxygen with a purity of in excess of 99.5 mol-% or as non-pure oxygenwith less purity, in particular with a purity of less than 98 mol-%.Along with a method of operation which is particularly favorable as faras energy is concerned, an arrangement that is as compact as possible isalso to be obtained.

Said object is achieved by the features of Claim 1. A precolumn, whichis known per se from WO 2009 095188 A2, is utilized in this case. Themethod of WO 2009 095188 A2 WO 2009 095188 A2, however, is chieflydirected toward the production of large volumes of oxygen under a veryhigh pressure of clearly in excess of 6 bar as a result of internalcompression. Pressurized nitrogen is certainly produced directly fromthe distillation column system in this case too, but this is onlypossible to a similarly small extent as in the case of the knownsecondary condenser method. The expert will therefore not at firstexpect to find a solution to the above-described technical problem in WO2009 095188 A2.

Only within the framework of the invention has it been shownsurprisingly that a method with a precolumn is not only suitable forinternal compression, but also in conjunction with a secondarycondenser, in which oxygen removed in the liquid state from thelow-pressure column is evaporated at an only slightly increased oxygenpressure, results in removal of a large volume of pressurized nitrogenwith a much higher yield. Over and above this, the method according tothe invention, in contrast to the process of WO 2009 095188 A2, ispreferably operated with a comparatively small amount ofpre-liquefaction of the air; the liquid part of the feed air which is tobe introduced into the distillation column system includes a maximum of29 mol-% and is in particular between 23 and 29 mol-%.

The secondary condenser, the head condenser and the precolumn arearranged one above the other. An arrangement of two elements “one aboveanother” is to be understood here as the upper end of the lower of thetwo elements being situated at a lower geodetic height than the lowerend of the upper of the two elements and the projections of the twoelements into a horizontal plane intersecting. For example, the twoelements can be arranged precisely one above another, this means theaxes of the two elements extend along the same vertical straight line.

In the case of the invention, three elements are arranged one above theother in the above-described sense such that overall a particularlycompact method of construction is produced.

The operating pressure of the precolumn is preferably chosen such thatit corresponds to the pressure of the second part stream of the airwhich is required for the oxygen evaporation in the secondary condenser.In particular, the operating pressure of the precolumn in the sump ispreferably between 7.5 bar and 10.5 bar.

The first gaseous nitrogen product can be removed under precolumnpressure (at the head of said column). Said pressure, in this case, isapproximately 9 bar. Depending on the desired end pressure, this meansthat a product compressor can be completely dispensed with or it can berealized with fewer stages than if the nitrogen product is removed fromthe low-pressure column or the high-pressure column. Within theframework of the invention, for example up to 30 mol-%, preferablybetween 5 and 25 mol-% of the feed air volume can be removed from theprecolumn as a first gaseous nitrogen product.

In the case of the invention, preferably all of the feed air iscompressed to the pressure of the precolumn (plus line losses) in themain air compressor (MAC). Consequently, there is no need for a boosterair compressor (BAC) that is driven by way of external energy. Inaddition, savings are produced in the investment costs as a result ofcorrespondingly smaller component parts in the “hot” part of the airseparation unit (precooling and purification device) and there is acomparatively smaller amount spent on regeneration in the purificationdevice.

The invention relates above all (but not only) to the area of relativelysmall units with extensive packaged units where dispensing withadditional compressors plays a key role both in the time and money spenton equipment and maintenance and on energy consumption. Thus, forexample, it is possible to dispense with small nitrogen boostercompressors which regularly have a relatively poor efficiency level.

Because in the case of the invention only a relatively small portion ofnitrogen is removed as a low-pressure nitrogen stream, the main heatexchanger has a correspondingly small volume and consequently there is afurther reduction in the time and money spent on equipment.

In a first variant of the invention, the secondary condenser is arrangedabove the precolumn and in a second variant it is arranged below theprecolumn.

In the case of the first variant, the secondary condenser and the headcondenser can be arranged in a common container. For example, thecontainer is realized as a vertical cylinder and comprises a tighthorizontal intermediate floor between the two apparatuses.

In addition, according to claim 2, a second gaseous nitrogen product canbe produced directly from the high-pressure column under, for example,between 5 and 6.5 bar, also without the use of a product compressor (andwithout low-pressure stages). This is particularly favorable when thecustomer requires nitrogen under different pressures which correspondapproximately to the operating pressures of the precolumn andhigh-pressure column. In addition, both nitrogen products can beproduced with different purities. In total (first and second gaseousnitrogen product) up to 50 mol-%, preferably between 25 and 50 mol-% ofthe feed air volume can be produced as a pressurized nitrogen product.

In the case of the invention, it is advantageous when cold is generated,according to claim 3, by a Claude turbine which is operated with a thirdpart stream of feed air and expands into the high-pressure column. Saidthird part stream is not fully cooled in the main heat exchanger (thatis not guided up to the cold end), but only up to an intermediatetemperature. The corresponding expansion machine is preferably formed byan expansion turbine. Said expansion turbine can be coupled to a boosterin which, in particular, the turbine stream (third part stream) isboosted upstream of the expansion for carrying out work.

In contrast to WO 2009 095188 A2, in the case of the method according tothe invention a smaller oxygen concentration in the raw oxygen fractionis preferably produced in the sump of the precolumn than in the sump ofthe high-pressure column. The two raw oxygen fractions are consequentlynot mixed with one another, but, according to claim 4, are fedseparately into the low-pressure column at different intermediatepoints. Between the two feed points there are, for example, between 5and 20, preferably between 7 and 15 theoretical floors.

The invention also relates to a device for the low-temperatureseparation of air according to claims 8 to 13.

The following variants are possible within the framework of theinvention and, where applicable, can also be combined together:

-   -   1. Arrangement of the precolumn next to a double column        (high-pressure column and low-pressure column one above the        other).    -   2. All columns are preferably accommodated in a prefabricated        rectification box. In order to utilize the area of the box in an        optimum manner, the secondary condenser is placed above the head        condenser of the precolumn.    -   3. All three columns side by side.    -   4. All the condenser-evaporators can be realized as single-story        bath evaporators (see exemplary embodiment below). Deviating        from this, other condenser-evaporator realizations can be used.        For example, the head condenser of the precolumn can be realized        as a forced-flow evaporator and/or the secondary condenser can        be realized as a reflux condenser with part liquefaction of the        second part stream of the air and/or the main condenser can be        realized as a multi-story bath evaporator (cascade evaporator).        The main condenser can also be realized as a falling film        evaporator with an associated circulating pump. Said pump can        also be combined with the oxygen product pump such that the        adjusting of the desired vapor content at the outlet out of the        falling film evaporator and the pumping of the product oxygen        into the secondary condenser are managed with only one single        pump.

The invention and further details of the invention are explained in moredetail below by way of three exemplary embodiments which are shownschematically in the drawing, in which:

FIG. 1 shows a first exemplary embodiment with a secondary condenserabove the head condenser,

FIG. 2 shows a second exemplary embodiment with a secondary condenserbelow the precolumn and

FIG. 3 shows a third exemplary embodiment with the arrangement of asecondary condenser and head condenser in a common container.

In FIG. 1, atmospheric air (AIR) is sucked in by a main air compressor202 via line 201 and compressed to a pressure of approximately 10 bar.The compressed feed air 203 is cooled in a precooling device 204 andthen purified in a purification device 205 which includes molecularsieve absorbers, that means water and carbon dioxide in particular areremoved.

The compressed and purified feed air 206 is cooled to a first part 210in a main heat exchanger 260 up to the cold end thereof. The “first partstream” 1 and the “second part stream” 2 a are formed from this. A“third part stream” 230 is recompressed in a booster compressor 466 withan aftercooler 467, guided via line 231 also to the hot end of the mainheat exchanger 260, only cooled there however to an intermediatetemperature and removed again. The cooled third part flow 232 isexpanded in an expansion turbine 465 so as to carry out work andforwarded via line 233. The expansion turbine 465 and the boostercompressor 466 are coupled in a mechanical manner.

In the case of the exemplary embodiment, the distillation column systemincludes a precolumn 10, a pressure column 11 and a low-pressure column12 as well as the condenser-evaporator linked thereto, the maincondenser 13 and the head condenser 14 of the precolumn. The secondarycondenser 46 is not part of the distillation column system. As anoption, the distillation column system can also comprise an argon partwhich includes, in particular, at least one raw argon column and itshead condenser; in addition, the argon part can comprise a pure argoncolumn for separating argon and nitrogen.

In the example, the separating columns for the separation of nitrogenand oxygen comprise the following operating pressures (in each case atthe head):

Precolumn 10 7.5 to 12 bar, for example 9.5 bar High-pressure column 5.0to 6.5 bar Low-pressure column 1.3 to 1.6 bar

The cooled gaseous (or somewhat pre-liquefied) first part stream 1 ofthe feed air from the cold end of the main heat exchanger 260 is under apressure which is just above the operating pressure of the precolumn 10and is introduced directly into the precolumn above the sump.

The precolumn 10 comprises a head condenser 14, into the liquefactionchamber of which a nitrogen stream 31 is introduced. A liquid secondpart stream 2 b of the feed air (see below) is introduced into theevaporation chamber of the head condenser 14 of the precolumn 10. Therest of the feed air is introduced into the distillation column system,in particular into the high-pressure column, via the line 233 in thegaseous state or substantially gaseous state. A gaseous stream 16 whichis enriched in oxygen is removed from the evaporation chamber of thehead condenser 14 and mixed with the gaseous air 233. As an alternativeto this, the streams 233 and 16 can be introduced separately (whereapplicable at different points) into the high-pressure column 11.

In the example, an additional liquid stream 4 is also directed into theevaporation chamber of the head condenser 14. This is formed by part ofthe sump liquid of the precolumn 10.

The remainder 5 a, 5 b of the sump liquid of the precolumn isundercooled here in an undercooling heat exchanger 37 and introducedinto the low-pressure column 12, at an intermediate point above thefeeding-in of the high-pressure column sump liquid 38. The liquid 6,which is generated from part 31 of the head nitrogen 30 of the precolumn10 in the condensation chamber of the head condenser 14, is fed as headreflux into the precolumn 10. Part 8 of the reflux liquid can be removeda little further down (as shown) and guided to the head of thehigh-pressure column 11.

The evaporated fraction 16 formed in the evaporation chamber of the headcondenser 14 is guided via line 17 to the sump of the high-pressurecolumn 11, together with the third part stream 233 of the feed air whichoriginates from the outlet of the Claude turbine 465. The flushingliquid 32 a, 32 b from the head condenser 14 of the precolumn 10 issupplied to the high-pressure column 11 at an intermediate point in thelower region.

Apart from this, the double column 11/12/13 functions in the generallyknown manner. From the high-pressure column 11, liquid raw oxygen 33 atthe sump and liquid non-pure nitrogen 35 from an intermediate pointrelatively high up are cooled in an undercooling heat exchanger 37 inindirect heat exchange with return streams and are introduced into thelow-pressure column 12 via the lines 38 or 40 at the suitable points.

The following products can be removed from the columns:

-   -   gaseous non-pure nitrogen 44, 45, 47 from the head of the        low-pressure column 12 (part thereof can be used as regeneration        gas in the purification device 205—not shown in the drawing).        Where required, a pure nitrogen portion can also be provided in        the low-pressure column 12 and low-pressure pure nitrogen can        also be produced,    -   liquid oxygen (a “liquid oxygen fraction”) 50 a from the sump of        the low-pressure column 12 gaseous pressurized nitrogen        (PGAN II) 51 a, 51 b from the head of the high-pressure column        11    -   gaseous nitrogen at particularly high pressure (PGAN I) 53 a, 53        b from the head of the precolumn 10.

The gaseous product streams are healed with feed air in the main heatexchange 260 indirect heat exchange. The main heat exchanger can consistof one block or of two or several blocks which are connected in paralleland/or in series. The oxygen 50 a removed in liquid form from thelow-pressure column is pressurized in liquid form in a pump 55 to apressure of, for example, between 2 and 5 bar, preferably between 2.7and 4.0 bar, and is then directed via line 50 b into the evaporationchamber of the secondary condenser 46. The evaporated oxygen 50 c isheated in the main heat exchanger to approximately ambient temperatureand is finally (50 d) produced as a gaseous oxygen product under mediumpressure (MP GOX). The second part stream 2 a of the feed air issubstantially fully liquefied in the liquefaction chamber of thesecondary condenser. The liquefied second part stream 2 b is introducedinto the evaporation chamber of the head condenser 14 of the precolumn10.

In order to produce a highly pure nitrogen product, many separationstages are needed in the corresponding column. In the case of the methodof the invention, it is particularly favorable to produce the highlypurified pressurized nitrogen from the precolumn as, in the case of thearrangement of columns and condensers shown in the drawing, the spaceabove the secondary condenser can still be utilized effectively. Theheight of the entire rectification coldbox is determined anyway by the(large) height of that of the double column part. The pressurizednitrogen product from the high-pressure column can then comprise a lowerpurity.

Both condensers 14 and 46 of the exemplary embodiments are realized asbath evaporators, at least one plate heat exchanger block being arrangedin a liquid bath.

FIG. 2 differs from FIG. 1 in that the secondary condenser 46 isarranged below the precolumn 10 and the head condenser 14.

FIG. 3 differs from FIG. 1 in that the secondary condenser 46 and thehead condenser 14 are arranged in a common container 301. The container301 is realized in a cylindrical manner and comprises a tightintermediate floor 302. Said variant is slightly more compact than thatof FIG. 1 and consequently needs less space. It also allows for evenmore cost-efficient production, more extensive prefabrication and easiertransport of the components.

1. A method for low-temperature separation of air in a distillationcolumn system which comprises a high-pressure column and a low-pressurecolumn, and where feed air is compressed in a main compressor, thecompressed feed air is purified in a purification device, the purifiedfeed air is cooled in a main heat exchanger, a first part stream of thecooled feed air is introduced into the distillation column system in agaseous state, a second part stream of the cooled feed air is introducedinto the liquefying chamber of a secondary condenser which is realizedas a condenser-evaporator with a condensation chamber and an evaporationchamber, a liquid oxygen fraction from the low-pressure column isintroduced into the evaporation chamber of the secondary condenser, anoxygen product fraction is removed from the evaporation chamber of thesecondary condenser in a gaseous state, is heated in the main heatexchanger and is finally obtained as a gaseous oxygen product and wherea first gaseous nitrogen product fraction is removed from thedistillation column system, is heated in the main heat exchanger and isobtained as a first gaseous nitrogen product, characterized in that thedistillation column system also comprises a precolumn, the operatingpressure of which is higher than the operating pressure of thehigh-pressure column, the first part stream of the cooled feed air isintroduced into the precolumn, the first gaseous nitrogen productfraction is removed from the precolumn, the precolumn comprises a headcondenser which is realized as a condenser-evaporator with acondensation chamber and an evaporation chamber, a liquefied portion ofthe second part stream is removed from the liquefaction chamber of thesecondary condenser and is introduced into the evaporation chamber ofthe head condenser, the secondary condenser, the head condenser and theprecolumn are arranged one above another, a gaseous fraction from theupper region of the precolumn is introduced into the condensationchamber of the head condenser and in that liquid formed in thecondensation chamber is fed to the precolumn at least in part as reflux.2. The method as claimed in claim 1, characterized in that the secondarycondenser is arranged above the precolumn.
 3. The method as claimed inclaim 1, characterized in that the secondary condenser and the headcondenser are arranged in a common container.
 4. The method as claimedin claim 1, characterized in that the secondary condenser is arrangedbelow the precolumn.
 5. The method as claimed in claim 1, characterizedin that a second gaseous nitrogen product fraction is removed from thehigh-pressure column, heated in the main heat exchanger and obtained asa second gaseous nitrogen product.
 6. The method as claimed in claim 1,characterized in that a third part stream of the cooled feed air isexpanded so as to carry out work and is introduced into thehigh-pressure column.
 7. The method as claimed in claim 1, characterizedin that a first raw oxygen stream is removed in a liquid state from thesump of the high-pressure column and is introduced into the low-pressurecolumn at a first intermediate point and in that a second liquid rawoxygen stream from the precolumn is introduced into the low-pressurecolumn at a second intermediate point which is arranged higher than thefirst intermediate point.
 8. A device for the low-temperature separationof air comprising a distillation column system, a high-pressure columnand a low-pressure column, wherein the distillation column system alsocomprises a precolumn, the operating pressure of which when the deviceis operating is higher than the operating pressure of the high-pressurecolumn, having a main air compressor for compressing feed air, having apurification device for purifying the compressed feed air, having a mainheat exchanger for cooling the purified feed air, having means forintroducing a first part stream of the cooled feed air into theprecolumn, wherein the precolumn comprises a head condenser which isrealized as a condenser-evaporator with a condensation chamber and anevaporation chamber, having means for removing a first gaseous nitrogenfraction from the precolumn, having means for heating the first gaseousnitrogen fraction in the main heat exchanger and for subsequentlyremoving it as a first gaseous nitrogen product, having means forintroducing a gaseous fraction from the upper region of the precolumninto the condensation chamber of the head condenser; having means forfeeding liquid that is formed in the condensation chamber into theprecolumn as reflux, having means for removing a first gaseous nitrogenproduct fraction from the precolumn, having a secondary condenser whichis realized as a condenser-evaporator with a condensation chamber and anevaporation chamber, wherein the secondary condenser, the head condenserand the precolumn are arranged one above another, having means forintroducing a second part stream of the cooled feed air into theliquefaction chamber of the secondary condenser, having means forintroducing a liquefied portion of the second part stream from theliquefaction chamber of the secondary condenser into the evaporationchamber of the head condenser, having means for introducing a liquidoxygen fraction from the low-pressure column into the evaporationchamber of the secondary condenser and having means for heating agaseous oxygen product fraction from the evaporation chamber of thesecondary condenser in the main heat exchanger and for subsequentlyremoving it as a gaseous oxygen product.
 9. The device as claimed inclaim 8, characterized in that the secondary condenser is arranged abovethe precolumn.
 10. The device as claimed in claim 8, characterized inthat the secondary condenser and the head condenser are arranged in acommon container.
 11. The device as claimed in claim 8, characterized bymeans for removing a second gaseous nitrogen fraction from thehigh-pressure column and by means for heating the second gaseousnitrogen fraction in the main heat exchanger and for subsequentlyremoving it as a second gaseous nitrogen product.
 12. The device asclaimed in claim 8, characterized by an expanding machine for expandinga third part stream of the cooled feed air so as to carry out work andby means for introducing the third part stream, which has been expandedso as to carry out work, into the high-pressure column.
 13. The deviceas claimed in claim 8, characterized in that a first raw oxygen line forremoving a first liquid raw oxygen stream from the sump of thehigh-pressure column and for introducing the first raw oxygen stream ata first intermediate point into the low-pressure column and by a secondraw oxygen line for removing a second liquid raw oxygen stream from theprecolumn and for introducing the second raw oxygen stream into thelow-pressure column at a second intermediate point which is arrangedhigher than the first intermediate point.